This invention relates to planographic printing and particularly, although not exclusively, relates to waterless lithographic printing.
Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas and image areas are arranged to have different affinities for printing ink. For example, non-image areas may be generally hydrophillic or oleophobic and image areas may be oleophilic. In "wet" lithographic printing, a dampening or fount (water-based) liquid is applied initially to a printing form prior to application of ink so that it adheres to the non-image areas and repels oil based inks therefrom. In "dry" or "waterless" printing, ink is repelled from non-image areas due to their release property.
Most lithographic printing at the present time is of the "wet" type and, therefore, requires application of a fount-solution during the printing process. However, lithographic printing forms for such processes may suffer from a number of disadvantages. Some of these are:
a) Providing the appropriate irk-water balance during press operation is difficult and requires great experience. If the correct ink-water balance is not achieved, "scumming" is caused wherein the printed ink image extends into the non-image areas thereby ruining the printed image. PA1 b) Adjustment of the ink-water balance at start-up or re-start is particularly difficult and cannot be stabilized until a large number of sheets have been printed, thus incurring waste. PA1 c) The ink tends to become emulsified which leads to poor adherence of the ink to ink-accepting areas of the printing form which causes problems in color reproduction and dot reproduction. PA1 d) The printing process has to be provided with a dampening system, thus increasing its size and complexity. In addition, dampening solutions contain undesirable volatile organic compounds. PA1 e) The printing form care chemistry and fount solutions require careful control and selection. In addition, printing form cleaners contain significant levels of solvent which is not desirable.
While waterless printing forms have been known for many years and potentially have advantages over forms of the "wet" type, there are few commercial waterless forms and those that are sold tend to be more expensive than those of the "wet" type.
One problem associated with waterless printing is that the printing forms become hot during printing and it is found that this tends to increase the adhesion of ink to non-image areas of the form and lead to scumming, with consequent reduction in the quality of prints typically produced. This problem is combatted on commercial presses by cooling the printing form and/or the ink during printing to maintain the temperatures within a narrow temperature band.
Currently, there is interest in imaging and/or developing printing forms directly on a printing cylinder of a printing press itself, thereby removing the need to image and/or develop a plate in a separate apparatus prior to attaching it to a press. For example, our co-pending patent application number GB 96 12233.8 describes a printing form that may be prepared directly on the press, by coating a hydrophillic surface of a support with a radiation sensitive ink; imaging the ink coating by digital laser means; and acting on the form with aqueous dampening rollers to remove unexposed areas of the ink coating to reveal the hydrophillic surface of the support and to leave an ink image formed from the ink which is oleophilic after exposure.
It is an object of the present invention to address problems associated with known planograrhic printing forms and methods for their preparation.